Acoustic imaging techniques are used in connection with visual inspection or analysis of all of the three phases of materials, i.e., solids, liquids and gases. Examples of applications of such techniques include industrial nondestructive evaluation in metals and liquids, medical ultrasound imaging, underwater imaging and echo ranging in the atmosphere. In the development of such techniques several classes of acoustic imaging systems have been constructed. These various classes of systems obtained display data in one, two or three dimensions. The various types of imaging devices include direct imaging systems such as acoustic cameras or pulse-echo devices and indirect reconstructed imaging devices including acoustical holographic devices, synthetic aperture and computed tomographic systems.
Acoustic imaging devices perform two basic tasks: data acquisition and data display. As stated, there are a variety of techniques by which these two functions are realized.
The most important one-dimensional imaging technique is referred to as pulse-echo A-mode in which a piezoelectric transducer transmits a short burst of acoustic energy into a medium and then receives and displays the amplitudes of echoes as a function of the echo range, i.e. the time of flight.
The most important two-dimensional acoustic imaging technique is pulse-echo B-mode tomography in which echoes returning to the transducer are displayed as brightness levels proportional to echo amplitude. The transducer is mechanically or electronically translated or steered in one-dimension. In the display the brightness levels are displayed with reference to echo range and transducer position or orientation providing cross-sectional images of the object.
Reference can be made to FIG. 1 of the drawings which will be used in describing the various prior art imaging techniques. This drawing includes a one-dimensional transducer array E-F which can produce a B-mode scan E-F-W-V of the object volume. B-mode images are comprised of many B-mode lines obtained at the rate of one B-mode line per transmitted acoustic pulse. The maximum B-mode line rate is given by: EQU R.sub.(lines/sec.) =v/2z
Where v is the acoustic propagation velocity and z is the maximum range of the image. Recent developments in this field have included parallel signal processing techniques which enable one to obtain and display several B-mode image lines per acoustic pulse.
A third important class of acoustic imaging systems in the prior art is the orthoscopic projection imaging system. Devices in this class include C-mode pulse-echo scanners, acoustic three-dimensional scanning systems, transmission or reflection acoustic cameras and acoustic holographic imaging systems. In these devices a three-dimensional volume of the object is interrogated by acoustic radiation via floodlight insonification or beam formed pulses. Data from the volume can be processed and displayed in several different ways.
For C-mode imaging systems a single transducer or array operates in the pulse-echo mode. The transducers are physically or electronically scanned through a rectangular raster format so that a three-dimensional volume of the object is interrogated by the ultrasonic beam, or the front surface of the transducer is fixed at a single point and the body of the transducer is moved in a spiral motion so that the transducer insonifies a conical three-dimensional volume in a spiral format. In each case only echo data from a preselected range is displayed as brightness levels proportional to echo amplitude. Due to the use of a fixed focus lens and an electronic range gate, a C-scanner presents two-dimensional data in an orthoscopic display in which the display coordinates are the x, y cartesian coordinates of the targets at a fixed depth in the object.
For example, referring to the diagram in FIG. 1 a single element of the two-dimensional transducer array A-B-C-D is fired and receives echoes from one line of the three-dimensional volume. Only those echoes which are located in a predetermined range gate are displayed in a single image point. After each element of the array has been fired subsequently, the complete C-mode image will be obtained, for example, in the plane R-S-T-U. Due to the fact that the display does not include target range, but includes directions perpendicular to target range, C-mode systems operate so that each point in the image requires a transmitted acoustic pulse. Thus, the time necessary to develop a complete C-mode image is significantly longer than the image formation time for a B-mode image, and an N.times.M point C scan requires N.times.M transmitted ultrasound pulses.
The prior art includes pulse-echo scanned three-dimensional imaging systems. One such system causes a transducer to be scanned in a raster format insonifying a three-dimensional rectangular parallelopiped. Cartesian coordinates are used in the display in a complicated manner which allows an orthographic display of a three-dimensional object in different projections, but without image perspective. Again referring to FIG. 1, the three-dimensional volume using this system is interrogated as in the case of the C-scan, but in this case the echoes from the entire volume are displayed as a function of the x,y coordinates so that parallel object planes in the z direction overlap in the images.
Another prior art system utilizes a three-dimensional scanner in which a conical volume is insonified by a combination of sector steering plus rotation. Again in this system Cartesian coordinates are used in the display, although it is claimed that some perspective view is obtained by modulating the size of the x,y display with the third Cartesian coordinate z.
In each of the types of three-dimensional scanning systems a line of pulse-echo data along one transducer orientation is displayed as a single point in the image. Accordingly, the time required to form the complete three-dimensional image is identical to the time required to develop a C-mode image. This amount of time is significantly greater than that required for the formation of a B-mode image. Therefore while these systems produce a greater amount of information the time required for the production of that information is similarly greatly increased.
It is therefore an object of this invention to provide an improved orthoscopic projection acoustic imaging system wherein an image of a three-dimensional volume having a greater amount of information can be realized in a much shorter time than with prior art techniques and wherein the acquisition and processing of acquired data occurs at a much faster rate.
Another object of this invention is to provide an improved acoustic imaging system of the foregoing type which utilizes the C-mode pulse-echo technique, but which uses parallel processing units to produce more than one image pixel for a transmitted acoustic pulse.
A further object of this invention is to provide a system of the foregoing type capable of producing the results described immediately above, but which is capable of operating using two dimensional transducer arrays or a one dimensional linear transducer array along with a mechanical displacement device.
Still another object of this invention is to produce an acoustic imaging of the foregoing type which provides range discrimination of targets and which incorporates colored coding to indicate range or amplitude weighting to enhance range discrimination.